Power supply for microphone

ABSTRACT

The invention concerns a circuit for the amplification of signals from a microphone, comprising a power source and a current generator which supplies a microphone, such as an electret microphone, with electrical energy in the form of pulses. The circuit clocks the power supply to the microphone with an active pulse time t 1 , and the sampling circuit reads the microphone signal in a window with the duration t 2  calculated from the rear flank of the active part of the supply pulse, whereby t 1  is shorter than the time period T corresponding to the sampling frequency 1/T, and whereby t 1  is of a length which is sufficient to enable the microphone current to reach a usable value, and whereby t 2  can be shorter than t 1.

The invention concerns a circuit for the amplification, analog signalprocessing and A/D conversion of signals from a microphone as defined inthe preamble to claim 1.

It is known within microphone and audio technology to integrate D/Aconversion and microphone amplification in one unit, so that thesampling point is moved as close as possible to the microphone, andherewith reduce signal distortion, noise and hum which can arise withlong signal paths. To reduce noise pulses, it is known from patentapplication GB-A-2 293 740 to build A/D converters and microphone powersupplies on the same circuit board, where the microphone power supplyworks with pulse modulation at a frequency which is derived from thesampling frequency in the A/D converter. This patent application formsthe basis for the two-part form of claim 1.

Where a wide range of portable products within telecommunication, videoand audiometrics are concerned, as well as hearing aids and othermicro-electronics, the weight and the physical dimensions of theequipment play an important role for the equipment's fields ofapplication and marketability.

The power consumption belongs typically among the important factorswhich, together with the relevant battery technology, are determinativefor precisely the weight and the physical dimensions of the portableequipment. Therefore, in many connections it is decisive that attemptsare made to reduce the power consumption as much as possible.

With active microphones, such as electret microphones, these arenormally supplied with a constant current which is in the magnitude of100-600 μA. For the above-mentioned applications, this constitutes ahigh current consumption. It is therefore a principle object of thepresent invention to reduce the current consumption.

This is achieved with the invention as defined in claim 1.

According to the invention as defined in claims 2-4, a strongly reducedcurrent consumption is achieved, in that the microphone coupling isprovided with current pulses of such a short duration that themicrophone current reaches a usable value. The current consumption insuch a coupling is typically only 0.01-0.03 μA per duty cycle.

According to the invention as defined in claim 5, a particularlyadvantageous coupling is achieved, in that the coupling together of themicrophone and amplifier in one unit makes a high signal/noise ratiopossible.

With reference to the figures, the invention will be described in moredetail-in the following, in that

FIG. 1 shows a principle diagram of the circuit,

FIG. 2 shows an example embodiment of the invention, and

FIG. 3 shows the signal sequences for the circuit according to theinvention.

In the principle diagram, FIG. 1, is shown an electret microphone which,for example, can have an upper limit frequency of around 15 kHz. Thisupper limit frequency can also lie closer to the maximum limit frequencyof the audible range if a microphone of high quality is used. Themicrophone can be protected by a thin protective net, such as a thinlayer of foam material which, however, will reduce the upper limitfrequency of the microphone membrane.

The membrane on an electret microphone comprises a variable capacitorwhich changes depending on the acoustic signal to which the microphoneis exposed. In the manufacture of the electret microphone, the membraneis provided with a permanent charge which can remain unchanged forseveral years. The equivalent diagram for an electret microphone canthus be considered as a battery in series with a variable capacitor.

In the principle diagram, FIG. 1, a microphone unit, MCU, comprises suchan electret microphone and a transistor, TMIC, which is placedphysically close to the membrane and connected to the membrane'sterminals. The transistor TMIC can with advantage be a J-FET transistorbecause of the ideal infinitely high input impedance of this type oftransistor. Small signals from signal sources with high output impedancecan hereby be amplified for further signal processing.

For the registration of the membrane movement, according to theinvention there is disclosed a voltage generator and possibly a currentgenerator for supplying the transistor TMIC in the microphone and thesubsequent signal processing with electrical energy. FIG. 1 shows avoltage generator and a current generator which are equivalent to anon-ideal impedance connected in parallel with a constant currentgenerator. This power supply has the designation SPL.

The object of the above-mentioned generators is to provide thetransistor TMIC with a constant operating current which is selected inaccordance with the optimum working specifications of the transistor.

A membrane deflection for a given time will give rise to a certainvoltage across the microphone membrane's terminals, which will result ina current which is proportional to the membrane deflection through thetransistor TMIC.

The constant working current is thus modulated by theacoustically-derived signal, so the current through TMIC varies aroundthe constant working current. It is this constant working current whichis desired to be reduced by the invention.

For reasons of cost, the current generator in the above-mentionedcoupling can be dispended with. However, this alternative will result ina lower signal/noise ratio, the reason being that the transistor doesnot work under ideal conditions.

According to the invention, the transistor TMIC is provided with currentacross an electric switch M1 which is controlled by a digital controlcircuit CTU via the signal MIC.PWR. This switch, M1, is opened andclosed at periodic intervals of T and is active for the time t1.

The voltage U_(mic) from the microphone supplies a sampling capacitor C5via the electric switch M2, which is active for the time t2 and iscontrolled by the signal MIC.SMPL from the control unit CTU. This signalis converted to digital values by a subsequent sampling circuit (notshown) which, synchronously with M1 and M2, operates at the samplingfrequency 1/T.

The sampling frequency or the Nyquist frequency can be selected in thenormal manner to be at least double the desired upper limit frequency ofthe audio signal. Sampling can also be effected in the conventionalmanner with over-sampling in order to reduce negative effects offiltration of the higher harmonic contributions from the samplingprocess.

It is also possible for the sampling process to be effected by a circuitworking analogically.

The time sequence of the signals MIC.PWR and MIC.SMPL is shown in FIG.3:

The time t1, where M1 conducts current to the transistor TMIC, isconsiderably shorter than the time period T, and is selected to be ofsufficient length for U_(mic) to reach a usable value. The microphoneamplifier is thus provided with relatively short pulses seen incomparison with the sampling time T.

Within the time t1, the output signal from the microphone is more orless constant, seen in relation to the variations within the time T, anda certain value higher or lower than at the last sample. This signalchange will now give rise to a change in the current through thetransistor TMIC.

Since in practice the microphone/transistor coupling MIC/TMIC containsparasite capacitances across the terminals, the current through thetransistor can not rise more quickly than that speed at which thesecapacitances can be charged and discharged. U_(mic) thus follows acharging or discharging sequence which converges asymptotically towardsa value which is proportional to the change of the given membranedeflection in relation to the last sample.

A typical sequence of U_(mic) is thus shown in FIG. 3.

The magnitude of the signal U_(mic), indicated by the stippled lines inFIG. 3, thus depends on the amplitude of the audio signal for a giventime.

The sampling circuit reads U_(mic) as late as possible within the timet1, the reason being that U_(mic) has the best signal/noise ratio at theend of t1. U_(smpl) is thus active in a window with the duration t2 seenfrom the rear flank of the active part of the supply pulse t1 controlledby M1. The time t2 is shorter than t1 and, depending on the speed atwhich C5 is charged, can be selected to be considerably shorter than t1.

U_(mic) can be considered as being more or less constant within the timet2, and the charging of the sampling capacitor C5 in the time t2 can beapproximated by an RC circuit in which R can vary from 500 ohms-5 Kohms,since the resistance of the electric switch M2 is insignificant. Typicalvalues for the time constant which applies during t2 will then be0.05-0.5 μs when C5 is of 100 pF.

The sampling capacitor C5 will thus be charged or discharged at theabove-mentioned time constant which applies during t2 from the previoussample value towards a level which asymptotically approaches the voltageacross the microphone membrane at a given time. This voltage, U_(smpl),is seen in FIG. 3.

How short t1 can be set in practice will depend on how low asignal/noise ratio can be accepted for U_(mic), which among other thingsmust be selected in accordance with the parasite capacitances arising inthe microphone transistor TMIC and with the accuracy of the samplingprocess and the use in general. It has proved in practice that acommencement of the sampling pulse (M2) already at t1-t2 correspondingto the double time constant (2 RC gives exp(−2RC/RC)=0.86) providesusable values. Typical values of t1 can lie at 0.2-3.0 μs.

If, for example, it is desired to transfer an audio signal of up to 20kHz, and a sampling frequency of 44 kHz is used (T=23 μs), it is seenthat the low values of t1 and t2 stated above will give rise to aconsiderable saving in current.

Speech signals can be transferred with acceptable results at a samplingfrequency of e.g. 10 kHz (T=100 μs), and in this case it is evident thatthe saving in current is even greater for the pulsed microphone circuit.

In FIG. 2 is seen an example embodiment where the current generator inFIG. 1 is configured with an operational amplifier OP1 which feeds thesignal U_(smpl) back through an electric switch M1 to the base of atransistor T1, which in turn supplies a microphone unit MCU (not shownin FIG. 2), which couples current to the terminal MIC.IND.

The operational amplifier is connected to the resistors R4, R5 and R6and the capacitor C3, which removes possible noise from OP1.

The transistor T1 is biased by the resistor network R1 and R2.

The output from the microphone unit can be damped via a capacitor asshown by C1 in order to avoid possible frequency contributions over thehalf sampling frequency being conducted further to the sampling circuit.

The signal from the microphone U_(mic) is fed across the electric switchM2, which in practice is connected to small parasite capacitances,forward to the sampling capacitor C5, across which there is coupled asubsequent A/D converter circuit with possible limiter circuit. M1 andM2 are controlled via the signals Mic_(pwr) and Mic_(smpl) by a controlcircuit CTU to operate as described above and synchronously with thesampling circuit SMPL.

The object of the coupling in FIG. 2 is to adjust or to adapt thecurrent through the microphone, so that a suitable average value for thevoltage across C5 is obtained. The voltage across C5 is controlled inaccordance with the adjustable level V_(bias), so that TMIC in themicrophone works at an optimized operation point.

The present invention is naturally not limited only to electretmicrophones as described in the example embodiment. The invention can beused with advantage for other types of active microphones, such ascapacitor microphones with external power source and piezo-sensitivesemi-conductor microphones. Similarly, other types of semi-conductorcomponents can be used instead of J-FET transistors.

A limiter circuit can be inserted in the signal path before the samplingcircuit. According to the invention, these circuit elements cansimilarly operate in a sampled manner and hereby further reduce thecurrent consumption.

Component list for the circuit in FIG. 2:

R1 470 ohms R2 330 ohms R4 15 Kohms R5 1 Megohm R6 47 Kohms C1 10 pF C310 μF C5 100 pF T1 BSR 20 A - BF 411 M1 IC 101 A - HC 4066 M2 IC 101 B -HC 4066 Op1 IC 102 B - HC 4066

What is claimed is:
 1. A circuit for the amplification of signals from amicrophone unit (MCU), comprising a power supply (SPL) which providesthe microphone unit (MCU) with electrical energy in the form of pulses,a sampling circuit for conversion of the microphone signal, in whichsampling is effected at a sampling frequency of 1/T, characterized inthat the power supply (SPL) transfers energy to the microphone unit(MCU) in the form of pulses with an active pulse time t1, and in thatthe sampling circuit reads the microphone signal in a window withduration t2 calculated from the rear flank of the active part of thesupply pulse, where t1 is smaller than the time period T correspondingto the sampling frequency 1/T, and where t2 is smaller than t1.
 2. Acircuit for the amplification of signals from a microphone unit inaccordance with claim 1, characterized in that t1 is at least 10 timessmaller than the time period T.
 3. A circuit for the amplification ofsignals from a microphone according to claim 1, characterized in that t2is at least 10 times smaller than t1.
 4. A circuit in accordance withclaim 1, characterized in that t1 is about 0.2 to 3.0 μs, t2 is about0.05 to 0.5 μs.
 5. A circuit for the amplification of signals from amicrophone according to claim 1, characterized in that the microphoneunit (MCU) comprises a microphone (MIC) and a transistor (TMIC), the oneterminal of which is connected directly to and placed close to themicrophone (MIC), whereby the transistor (TMIC) is supplied with currentthrough a first switch (M1) which connects the current from the powersupply (SPL) for the time t1, and whereby an output signal which isamplified in the transistor (TMIC) is transferred to the subsequentsampling circuit by a second switch (M2) which is closed for the timet2, and whereby the switches (M1, M2) are controlled by a control unit(CTU).